The hot process softener is a chemical reaction unit for treating a liquid, such as boiler feedwater, with chemicals at an elevated temperature in order to reduce dissolved and suspended impurities to low values. Such units are extremely versatile and are capable of treating raw waters of wide ranges of composition to produce an effluent which meets present-day stringent requirements for minimum concentrations of hardness, silica, oxygen, alkalinity and dissolved solids in a feedwater.
The typical hot process softener unit includes a vessel defining an upper reaction chamber and a lower settling chamber. The liquid to be treated is directed into the reaction chamber wherein its temperature is elevated by steam directed thereinto and it is mixed with chemicals and recirculated sludge, to promote chemical reaction and settling of suspended solid particulates. The liquid mixed in the reaction chamber is directed through a downcomer tube into a separation zone within the settling chamber so as to cause solid particulates to settle into a sludge collection zone and clarified liquid to rise within the separation zone to be withdrawn therefrom as treated liquid.
It is the present practice to merely spray the raw liquid to be treated, chemicals and recirculated sludge into the reaction chamber and rely on residence time to form the mixture which is directed into the settling chamber. As a consequence, hot process softening units have historically operated at flow rates significantly below the theoretically possible flow rates through the settling chamber. Such hot process softening units have not made efficient use of the reaction chamber volume and the settling chamber volume. Further, the efficiency of chemical usage and silica reduction has not been maximized.
Accordingly, there is a need for a hot process softening unit design which makes more efficient use of the reaction chamber volume and thereby permits the unit to operate at a flow rate which more closely approaches the theoretical flow rate of the separation chamber volume. There is also a need for a hot processs softening unit design which improves chemical usage efficiency and silica reduction.